The present invention relates generally to optical devices. More particularly, the present invention relates to an optical cavity having micron-sized dimensions and suitable for creating optical interference effects.
Optical cavities suitable for generating controllable optical interference effects (hereinafter simply xe2x80x9coptical cavitiesxe2x80x9d) are known and used in the art. Such controllable optical interference effects have been used, for example, to modulate, attenuate, equalize and filter optical signals. Optical cavities are thus a key operative element of a variety of optical devices.
One type of optical cavity, known as the Fabry-Perot etalon, normally consists of two high and equal reflectivity dielectric mirrors that are separated by a gap. The mirrors, which are usually aligned so that they are parallel, have a nominal thickness equal to one-quarter of a wavelength of the optical signal being processed. The etalon exhibits a reflectivity (e.g., a controllable optical interference effect) that is a function of the distance between the mirrors (i.e., the gap) and the properties of the mirrors.
Typically, one of the mirrors of the etalon is movable while the other mirror is non-movable. The movable mirror is moved, in most cases, by applying a voltage to electrodes that overlie and underlie the movable mirror. As the movable mirror is urged towards the non-movable mirror, the gap changes. In such manner, the reflectivity of the etalon is controllably varied.
With the advent of micromachining techniques, devices with optical cavities having dimensions measured in microns have been fabricated. The optical cavity of such micron-sized devices is usually formed via surface micromachining technologies using layers of silicon nitride or polysilicon. Typical of such surface micromachining technologies are those offered by CHRONOS (formerly the MEMS Microelectronics Center of North Carolina).
CHRONOS offers, among other processes, a three-polysilicon-layer surface micromachining process. The first layer (POLY0) of the three polysilicon layers is xe2x80x9cnon-releasablexe2x80x9d (i.e., remains non-movable) and is used for patterning address electrodes and local wiring on a substrate, such as a silicon wafer. The other two polysilicon layers (POLY1 and POLY2) are xe2x80x9creleasablexe2x80x9d (i.e., can be rendered movable) and so can be used to form mechanical structures (e.g., movable mirror elements, etc.).
Release is achieved by etching away sacrificial material, typically an oxide layer, that is deposited between the POLY1 and POLY0 layers or the POLY1 and POLY2 layers. Etching produces a thin membrane or layer of polysilicon that xe2x80x9cfloatsxe2x80x9d above the underlying polysilicon layer, defining a cavity therebetween. The floating or xe2x80x9creleasedxe2x80x9d portion is potentially movable (e.g., via application of a voltage). When appropriately dimensioned, such an arrangement is suitable for creating a Fabry-Perot etalon or other types of optical cavities.
Companies that manufacture products incorporating such optical cavities may use their own xe2x80x9cin-housexe2x80x9d fabrication processes, rather than a commercial MEMS foundry such as CHRONOS, to form suspended structures that function as movable mirrors. Such in-house processes typically involve the deposition and selective removal, typically via photolithographic techniques, of various polysilicon, silicon nitride and sacrificial (e.g., oxide) layers.
Whether obtained from outside suppliers or made in-house, optical cavities and methods for their fabrication, as described above, suffer from a variety of drawbacks. Several of such drawbacks are discussed below.
A first of such drawbacks is related to materials selection. In particular, the polysilicon or silicon nitride layer serving as the movable mirror is typically a xe2x80x9cstressedxe2x80x9d layer (tensile stress for silicon nitride and compressive stress for polysilicon). The stress results from temperature cycling that occurs during deposition/growth of polysilicon or silicon nitride on the underlying sacrificial layer (e.g., silicon dioxide, etc.) since the sacrificial layer necessarily has a coefficient of thermal expansion that is different than that of the overlying polysilicon or silicon nitride layer.
The mechanical properties of the movable mirror/resonant cavity, such as its speed and switching voltage, are dependent upon the level of stress in the movable mirror, with higher stress resulting, at least potentially, in a greater speed. Unfortunately, such stress tends to cause released structures to xe2x80x9ccurlxe2x80x9d or xe2x80x9cwarp,xe2x80x9d resulting in unpredictable device response and reliability problems.
Furthermore, prior art xe2x80x9creleasexe2x80x9d type processes for forming optical cavities typically rely on a timed wet etch to remove sacrificial layers that underlie the layer that is to form the movable layer (mirror). Deviations in the timing of the etching step will change the size (i.e., the diameter) of the movable mirror. The size of the movable mirror, and thus the ability to repeatably produce a cavity having a particular response, is dependent to a high degree on a precisely timed etch.
Optical cavities that have a movable mirror formed from silicon nitride, which is an electrical insulator, must have an electrode deposited thereon. The resulting structure thus has an insulator (i.e., silicon nitride) sandwiched by two conductors (i.e., the overlying electrode and the underlying electrode). During operation, charge tends to accumulate on the lower surface of the silicon nitride layer. Since it is difficult to fully discharge the silicon nitride layer, control of the movable mirror becomes problematic.
The art would therefore benefit from improved fabrication methods and improved optical cavities resulting therefrom.
Some embodiments of the present invention provide an optical cavity for generating optical interference effects without some of the disadvantages of the prior art. In particular, some embodiments of the present optical cavity, and a method for forming such a cavity:
do not incorporate a stressed layer;
do not require a timed etch; and
do not have an insulator between electrodes.
The present optical cavity, and improved optical devices comprising such a cavity in accordance with the present teachings, include two mirrors, one of which is advantageously movable, that are spaced from and parallel to one another. In some embodiments, both mirrors are formed from unstressed single crystal silicon.
The single crystal silicon used in some embodiments of the present invention is advantageously sourced from single crystal silicon-on-insulator (hereinafter xe2x80x9cSOIxe2x80x9d) wafers. Such wafers include a buried insulator layer that is sandwiched between two layers of silicon. One of the layers is a xe2x80x9cthinxe2x80x9d (i.e., 0.1 to 1.5 microns) layer of silicon that is used for forming movable and non-moving mirrors. The other of the two layers is a xe2x80x9cthickxe2x80x9d layer of silicon (i.e., 0.5 to 0.7 millimeters).
In an illustrative method in accordance with the present invention, a non-moving mirror is patterned in the thin silicon layer of a first SOI wafer. A standoff is disposed on the thin silicon layer near the mirror. The thin silicon layer of a second SOI wafer is attached to the standoff such that there is a gap between the two, spaced, thin silicon layers.
The thin silicon layer of the second SOI wafer is released forming a movable mirror by removing the thick layer of silicon and the buried oxide from that wafer. The movable mirror, the non-moving mirror patterned in the first SOI wafer, and the gap therebetween define an optical cavity.
In single crystal SOI wafers, the thin silicon layer that forms the movable mirror is xe2x80x9czeroxe2x80x9d stress, thus avoiding the warping/curling problems of prior art stressed layers. Furthermore, as the various layers that require removal for defining the cavity or releasing the movable mirror are not xe2x80x9cburiedxe2x80x9d (i.e., do not underlie layers that must be preserved), a wet etch is not required. Rather, reactive ion etching is advantageously used for such etch steps, thereby avoiding the critical timing issues of a wet etch. Since the silicon mirrors are conductive (the mirrors are usually doped to improve such conductivity), the charge accumulation problem experienced with silicon nitride mirrors is avoided.